The American Society of Anesthesiologists (ASA) Task Force on Management of a Difficult Airway defines a difficult airway as the clinical situation in which anticipated or unanticipated difficulty or failure is experienced by a physician trained in anesthesia care, including difficulty with one or more of the following: face mask ventilation, laryngoscopy, ventilation using a supraglottic airway (SGA) device, tracheal intubation, extubation, or invasive airway. Difficulty with airway management can lead to hypoventilation, hypoxemia, or soiling of the tracheobronchial tree.

The prevalence of difficult face mask ventilation is dependent on the definition. In one study, 5% of 1502 nonpregnant patients experienced difficulty in face mask ventilation, which was defined as an oxyhemoglobin saturation < 92% when using F io ₂ 1.0. Impossible face mask ventilation , defined as an inability to exchange air during bag-and-mask ventilation despite multiple providers, airway adjuncts, and neuromuscular blockade, was reported in 77 of 50,000 (0.15%) nonobstetric anesthetic procedures. Difficult laryngeal mask ventilation may be defined as the inability after three attempts of device insertion to produce expired tidal volumes > 7 mL/kg (leak pressure > 15 to 20 cm H ₂ O). In a study of 11,910 nonobstetric patients, the incidence of difficult laryngeal mask ventilation was 0.19%.

The definition of failed tracheal intubation varies widely, ranging from tracheal intubation not accomplished with a single dose of succinylcholine to inability to intubate during general anesthesia. Such variation inevitably leads to heterogeneity in the reported rate of failed intubation. Defining difficult tracheal intubation is even more complex. Difficulty may be encountered because of failure to visualize the glottis (difficult laryngoscopy) or due to an anatomic laryngeal or tracheal abnormality. Difficulty has been variously defined by (1) the time required to intubate, (2) the number of attempts, (3) the view at laryngoscopy, and (4) the requirement for special equipment or procedures.

Although a dramatic decrease in the number of anesthesia-related maternal deaths has been reported in the UK Confidential Enquiries into Maternal Deaths over the past 40 years, complications from general anesthesia, primarily in the form of complications of airway management, continue to be a leading cause of anesthesia-related maternal mortality. Similarly, data from the United States have demonstrated ongoing risks of failed airway management in obstetric anesthesia. Although the development of national guidelines has resulted in a more systematic approach to the management of the difficult airway, deaths directly resulting from anesthesia still occur due to failures in ventilation, tracheal intubation, or airway management following extubation. Despite the widespread use of neuraxial anesthesia for operative delivery, general anesthesia may still be required in emergency situations or in the setting of contraindications to neuraxial anesthesia, patient refusal, or, most commonly, inadequate neuraxial anesthesia.

The incidence of failed intubation in obstetrics has long been considered to be approximately 1 in 250 to 300 ( Table 29.1 ). A 2015 systematic review including data from published studies, abstracts, and databases reported a failed intubation rate at cesarean delivery of 1 in 433. The incidence of failed intubation for all obstetric procedures was slightly higher at 1 in 390, which likely reflects intubation difficulties resulting from airway swelling following crystalloid resuscitation in postpartum hemorrhage. The authors noted no increase in incidence from 1985 to 2014. However, there was significant heterogeneity among reports. In 2022, a large, multicenter, retrospective investigation reported a difficult intubation rate of 1 in 49 based on an analysis of nearly 15,000 cases of cesarean delivery performed under general anesthesia in 45 medical centers enrolled in the Multicenter Perioperative Outcomes Group. Difficult intubation was defined as Cormack-Lehane view grade 3 ≥, three or more intubation attempts, or the requirement of rescue fiberoptic intubation, rescue SGA, or surgical airway. Failed intubation was defined as any attempt at intubation without successful endotracheal tube (ETT) placement; the rate of failed intubation was 1 in 808.

The failed intubation rate in obstetric patients is approximately eight times higher than estimates of the rate in nonobstetric surgical populations. A number of reasons have been proposed to explain the increased difficulty with obstetric airway management. Significant physiologic and anatomic changes of pregnancy (see later discussion) affect the airway, oxygenation, and metabolism. The majority of obstetric general anesthetics are administered for emergency deliveries, often during off-hours, when there are likely to be fewer personnel available for assistance. Excessive cricoid pressure applied by a poorly trained assistant can worsen the glottic view at laryngoscopy, as can positioning the parturient with left lateral tilt. With an increasing focus on neuraxial anesthesia and a concomitant decrease in the number of cesarean deliveries performed under general anesthesia, trainees also have fewer opportunities to become familiar with challenges of the obstetric difficult airway.

Changes in maternal characteristics, most notably an increase in the prevalence of maternal obesity, may increase the risk for complications from general anesthesia, especially when performed for emergency procedures. Women with obesity are at increased risk for obstetric interventions requiring anesthesia and are also at increased risk for failed neuraxial anesthesia, necessitating the use of general anesthesia for emergency delivery (see Chapter 48 ). In the large Multicenter Perioperative Outcomes Group cohort, a body mass index (BMI) ≥ 40 kg/m ² was found to be associated with a 1 in 28 risk of difficult intubation, and had an odds ratio of 2.02 for difficult intubation compared with women with a BMI < 25 kg/m ² . Poor head and neck positioning at induction of anesthesia, inappropriately applied cricoid pressure, macromastia, and shorter interval from start of apnea until significant oxygen desaturation may be responsible for a higher incidence of difficult airway management in obese patients.

The presence of experienced anesthesia staff during induction of general anesthesia is recommended to reduce the morbidity and mortality, and perhaps the frequency, of difficulty with airway management. The introduction and widespread acceptance of simulation training in obstetrics may also lead to improvement in staff performance during critical events such as difficult airway management (see Chapter 11 ). It is critical to maintain situational awareness and recognition of the risk for fixation error, especially when dealing with emergency airway issues and the perceived need for tracheal intubation.

Given the multitude of risks inherent in general anesthesia, along with the maternal morbidity associated with the difficult and failed airway management, avoiding general anesthesia when possible is critically important (see Chapter 26 ). The rate of general anesthesia for cesarean delivery is a notable quality and safety metric; the Society for Obstetric Anesthesia and Perinatology Centers of Excellence criteria call for an overall rate of general anesthesia less than 5% of all cesarean deliveries. The Royal College of Anaesthetists has recommended a threshold greater than 99% for conversion of neuraxial labor analgesia to neuraxial anesthesia for elective cesarean delivery, greater than 95% for urgent deliveries, and greater than 85% for emergency deliveries. When general anesthesia rates are high, it is likely that at least some of the general anesthetics are avoidable with high-quality obstetric anesthesia care.

In an analysis of 60,000 general anesthetics for cesarean delivery, Guglielminotti et al. deemed 44% of general anesthetics to be avoidable; these cases had no known indication for general anesthesia or contraindication to neuraxial anesthesia. Avoidable general anesthetics were associated with an increased risk of anesthesia-related complications, surgical site infection, and venous thromboembolism, suggesting that an overreliance on general anesthesia is associated with multiple poor processes that compromise maternal safety. The authors found that low hospital-level rates of neuraxial analgesia were among the strongest predictors of avoidable general anesthesia.

To decrease the frequency of avoidable general anesthesia, several strategies are necessary. High rates of neuraxial labor analgesia use are required, along with successful conversion of neuraxial analgesia to surgical anesthesia for cesarean delivery (see below discussion). Close communication among anesthesia providers, obstetricians, midwives, and labor and delivery nurses maximizes the likelihood of successful conversion of epidural analgesia to intrapartum cesarean delivery anesthesia. Frequent multidisciplinary communication is necessary to allow the anesthesia provider to understand the likelihood of emergency cesarean delivery and to plan for appropriate anesthesia techniques in these eventualities. Furthermore, regular collaboration between obstetric teams and obstetric anesthesia providers throughout the peripartum period to discuss high-risk parturients may also help reduce rates of general anesthesia.

Although rare, maternal morbidity from airway-related issues remains an important problem. In the report covering the 2012 to 2014 triennium, the UK Confidential Enquiries into Maternal Deaths reported that pregnancy-related mortality from anesthetic causes was the 11th most common cause of maternal mortality, accounting for 1% of maternal deaths. In the United States, 1.6% of maternal deaths between 1991 and 2002 were related to complications of anesthesia care, representing a 59% reduction in anesthesia-related mortality compared with data from 1979 to 1990. These data demonstrate dramatic improvements in anesthesia-related maternal mortality in the past three decades, likely reflecting efforts by national anesthesia organizations in defining standards of care that lead to improved maternal safety.

Compared with neuraxial anesthesia, general anesthesia has historically been associated with a greater risk for maternal mortality ( Table 29.2 ; see Chapter 39 ). Using data from the Centers for Disease Control and Prevention from 1985 to 1990, the estimated case-fatality risk ratio for general anesthesia versus neuraxial anesthesia was 16.7. However, the estimated risk ratio for the period between 1997 and 2002 was only 1.7 (95% confidence interval [CI], 0.6 to 4.6; P =.2). Improvements in monitoring and rescue airway equipment, and the publication of algorithms for difficult airway management, may account for the reduction in mortality from general anesthesia over time. Unfortunately, maternal deaths directly attributable to general anesthesia are still reported, and although protocols for the management of a difficult airway are now widespread, they may not be uniformly followed.

Hypoventilation and airway obstruction after extubation are now increasingly recognized as causes of maternal mortality. In Michigan between 1985 and 2003, eight maternal deaths were believed to be related to anesthesia care; all deaths occurred during emergence from general anesthesia or the recovery period, and six of the eight patients were obese. System errors in which the care of the patient did not meet recognized standards were identified in five of the eight cases. These errors included inadequate supervision by an anesthesiologist and lapses in postoperative monitoring.

Many anatomic and physiologic changes occur in pregnancy (see Chapter 2 ) and several have significant effects on the difficult airway ( Box 29.1 ).

Respiratory and Metabolic Changes

As pregnancy progresses, the gravid uterus increasingly encroaches on the diaphragm and lung volumes are reduced (see Chapter 2 ). By term, expiratory reserve volume decreases by 25% and residual volume decreases by 15%, resulting in a 20% reduction in functional residual capacity (FRC). This decrease is most notable with the patient in the supine position, as well as in the obese patient. Closing volume is unchanged in pregnancy, but the decrease in FRC results in airway closure in 50% of women in the supine position. Metabolic requirements for oxygen increase by nearly 60% during pregnancy, predominantly because of fetal demands. This oxygen requirement is further increased during labor. Taken together, these changes contribute to a greater likelihood that pregnant women will become hypoxemic during periods of apnea, such as during the induction of general anesthesia. Therefore, adequate denitrogenation/preoxygenation is vital to delay the onset of hypoxemia during periods of apnea (see later discussion).

Preoxygenation and the rate of hemoglobin desaturation have been investigated by computer modeling. In these models, labor, morbid obesity, and sepsis all hasten preoxygenation; however, desaturation also occurs more rapidly in moderately ill and obese women ( Fig. 29.1 ). Importantly, the time to life-threatening hypoxemia is significantly shorter than that for recovery from paralysis from succinylcholine. Therefore, should ventilation be impossible, it cannot be assumed that the patient will recommence breathing before dangerously low levels of oxygen saturation have been reached. If rocuronium is used instead of succinylcholine, and ventilation and oxygenation are impossible, emergency reversal of neuromuscular blockade with sugammadex 16 mg/kg is recommended and may allow for reversal of neuromuscular blockade faster than spontaneous recovery of ventilation after succinylcholine 1 mg/kg administration. Studies of emergency sugammadex reversal in obstetric patients are currently lacking.

Time course of Sa o ₂ during apnea in pregnant (horizontal hatching) and nonpregnant (vertical hatching) virtual subjects predicted to tolerate short, average, and long periods of apnea. Heavy lines represent average subjects. (a) Pregnant subjects, no preoxygenation; (b) nonpregnant subjects, no preoxygenation; (c) pregnant subjects, 99% complete denitrogenation; (d) nonpregnant subjects, 99% complete denitrogenation.

From McClelland SH, Bogod DG, Hardman JG. Apnoea in pregnancy: an investigation using physiological modelling. Anaesthesia . 2008;63:264–269.

Cormack and Lehane devised a glottic view grading system in 1984; the original purpose of the system was to grade the glottic view obtained with direct laryngoscopy to help train for difficult airways during obstetric general anesthesia. Thus, the Cormack and Lehane grade is a classification method to describe the relative difficulty with subsequent tracheal intubation. The original description includes four grades of laryngoscopy ( Fig. 29.2 ):

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  Grade 1: Full view of glottis

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  Grade 2: Partial view of glottis or arytenoids

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  Grade 3: Only epiglottis visible

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  Grade 4: Neither glottis nor epiglottis visible

Cormack and Lehane Laryngoscopic View Grades.

Grade I is visualization of the entire laryngeal aperture. Grade II is visualization of only the posterior portion of the laryngeal aperture. Grade III is visualization of only the epiglottis. Grade IV is visualization of only the soft palate.

From Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia . 1984;39:1105–1111.

Subsequent modifications have been proposed: grade 2 may be divided into 2A (part of vocal cords visible) and 2B (only arytenoids or very posterior origin of vocal cords visible). Grade 3 may be divided into those in whom the epiglottis is visible and is able to be lifted, such as with a gum elastic bougie (Grade 3A), and those in which the epiglottis is visible but not able to be lifted (Grade 3B).

Because of the widespread acceptance of the Cormack and Lehane grading system, useful information about the potential for difficulty with intubation can be gained by reviewing previous anesthetic records. However, prior reports should be treated with caution because grades given in the nonpregnant state will likely differ from those determined during pregnancy, and the potential for interobserver and intraobserver variability exists.

In 1985, Mallampati et al. described a 3-point scale of the oropharyngeal view of the open mouth; the more the view was obscured, the greater the difficulty with laryngoscopy and intubation. Samsoon and Young later modified the scoring system into a 4-point scale ( Fig. 29.3 ):

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  Class I: Visualization of soft palate, uvula, and tonsillar pillars

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  Class II: Visualization of soft palate and base of uvula

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  Class III: Visualization of soft palate only

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  Class IV: Visualization of hard palate only

Modified Mallampati Classification of the Oropharynx.

Classification of the upper airway in terms of the size of the tongue and the pharyngeal structures that are visible with the mouth open. In class I, the soft palate, uvula, and anterior and posterior tonsillar pillars can be seen. In class II, the soft palate and uvula can be seen; the tonsillar pillars are hidden by the tongue. In class III, the soft palate and the base of the uvula can be seen. In class IV, only the hard palate can be seen.

From Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict difficult tracheal intubation: a prospective study. Can Anaesth Soc J . 1985;32:429–434.

The test should be performed with the patient sitting upright with her head in the neutral position; the patient should open her mouth as wide as possible and protrude her tongue as far as possible without phonation. Increasing difficulty with laryngoscopy and tracheal intubation has been demonstrated with greater Mallampati scores in both obstetric and nonobstetric populations.

Mallampati scores are frequently used as part of an assessment to predict difficult intubation. However, scores change both during pregnancy and during labor. When used as the sole predictor of a difficult airway, the incidence of both false-positive and false-negative results is high due to the use of phonation, poor patient positioning, involuntary arching of the tongue, and interobserver variability in interpretation. A metaanalysis of the Mallampati score concluded that the test had limited accuracy for predicting a difficult airway and was not a useful screening test, and as such it is best used in combination with other tests.

During laryngoscopy, the tongue is normally pushed into the mandibular space. The thyromental distance, the distance from the tip of the chin to the notch of the thyroid cartilage, can be used to estimate the volume of this space and whether the tongue can easily be displaced to facilitate laryngoscopy. A thyromental distance < 6 cm suggests an increased risk for airway difficulty.

Extension of the atlanto-occipital joint is necessary for the patient to be in the ideal intubating position in which the oral, pharyngeal, and laryngeal axes are aligned (see later discussion). Movement can be assessed with the patient seated with the head and neck in the neutral position facing forward and then with the joint maximally extended ( Fig. 29.4 ). Normal extension should be 35 degrees or more; difficulty with intubation can be expected when joint movement is decreased.

Clinical Method for Quantifying Atlanto-occipital Joint Extension.

When the head is held erect and faces forward, the plane of the occlusal surface of the upper teeth is horizontal and parallel to the floor. When the atlanto-occipital joint is extended, the occlusal surface of the upper teeth forms an angle with the plane parallel to the floor. The angle between the erect and the extended planes of the occlusal surface of the upper teeth quantifies the atlanto-occipital joint extension. A normal person can produce 35 degrees of atlanto-occipital joint extension.

From Bellhouse CP, Dore C. Criteria for estimating likelihood of difficulty of endotracheal intubation with Macintosh laryngoscope. Anaesth Intensive Care . 1988;16:329–337.

The patient’s ability to extend the mandibular teeth anteriorly beyond the line of the maxillary teeth may predict adequate visualization of the larynx during direct laryngoscopy. In the mandibular protrusion test, patients are asked to protrude their mandible as far as possible ( Fig. 29.5 ); one of three classes is assigned

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  Class A: The lower incisors can protrude anterior to the upper incisors

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  Class B: The lower incisors can be brought edge to edge with the upper incisors

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  Class C: The lower incisors cannot be brought edge to edge with the upper incisors

Mandibular Protrusion Test.

Three classifications are based on the test. Class A predicts a good glottic view with direct laryngoscopy, whereas class C is associated with a poor view.

Redrawn from Munnur U, de Boisblanc B, Suresh MS. Airway problems in pregnancy. Crit Care Med . 2005;33:S259–S268.

Class A predicts a good glottic view with direct laryngoscopy, whereas class C is associated with poor glottic view.

The upper lip bite test (ULBT) is similar to mandibular protrusion. In class I, the lower incisors can bite the upper lip above the vermillion border; in class II, the lower incisors can bite the upper lip below the vermillion border; and in class III, the lower incisors cannot bite the upper lip. The ULBT has been shown to be a better predictor than a Mallampati score for predicting ease with laryngoscopy and intubation.

Limited mouth opening impedes the introduction of a laryngoscope blade or other airway devices; an interincisor distance < 5 cm may predict difficult intubation. Mouth opening < two fingerbreadths has been shown to reduce the prevalence of easy intubation from 95% to 62%. Mouth opening also is influenced by cervical spine movement; if movement is limited, mouth opening may also be restricted. Protruding maxillary incisors, a single maxillary incisor, and missing maxillary incisors have been shown to be predictive of difficult intubation in obstetric patients. Components of a comprehensive preoperative airway evaluation are listed in Table 29.3 .

Individual tests are poorly predictive of airway difficulty; therefore, investigators have combined assessments in an effort to improve specificity. In a study of 400 pregnant women scheduled for elective cesarean delivery, Honarmand and Safavi evaluated Mallampati class score, ratio of height to thyromental distance, and the ULBT, both in isolation and combination. A total of 8.75% of patients had a Cormack and Lehane grade 3 or 4 laryngoscopic view; the ratio of height to thyromental distance was the best predictor of this outcome. Reale et al. assessed the following factors for a composite risk of difficult intubation: age ≥ 35 years, BMI ≥ 25 kg/m ² , ASA physical status score III or IV, Mallampati score III or IV, small hyoid-to-mentum distance, limited jaw protrusion, limited mouth opening, altered neck anatomy, cervical spine limitations, and presence of preeclampsia or eclampsia. The frequency of difficult intubation increased from 0.8% for women with one risk factor for difficult intubation, to 3.7% for women with three risk factors, to 8.8% for women with five or more risk factors.

The thoroughness of the airway assessment may be limited by the urgency of surgery. For emergency procedures, relatively little time is available; thus, all women in the obstetric unit should be assessed soon after their arrival, focusing on those with the greatest number of risk factors for difficult intubation and those with the greatest risk for surgical intervention. However, changes in assessment during the course of labor must be anticipated, and reevaluation before inducing anesthesia is vital to the safe care of these patients.

The assessment should start with a history and physical examination to detect factors that may indicate the presence of a difficult airway, as well as the potential risk for pulmonary aspiration. Examination of previous anesthetic records, if available, may indicate problems with ventilation or intubation. Comorbidities may influence airway management and should be considered before anticipated airway management. Most notably, maternal obesity is associated with an increased incidence of airway problems (see earlier discussion). Similarly, difficulties in airway management should be anticipated in patients with preeclampsia with severe features, or those who receive large volume fluid resuscitation for obstetric hemorrhage.

Asssessing and documenting mouth opening, the Mallampati class, atlanto-occipital mobility, thyromental distance, and mandibular protrusion may be performed relatively quickly and should identify most patients who will present difficulties with airway management. When risk factors are identified, appropriate plans for airway management, such as the availability of additional equipment and personnel (e.g., individuals experienced with airway management and the creation of a surgical airway) should be made. The proposed plan should consider that the administration of a neuraxial anesthetic technique may be the safest option for both mother and infant, even in the presence of nonreassuring fetal status. The use of ultrasonography to identify the cricothyroid membrane before the induction of general anesthesia may improve landmark identification should emergency surgical (front-of-neck) access be required to rescue failed tracheal intubation. The risks and benefits of various alternatives should be discussed with the patient and the obstetric, neonatal, and nursing teams and documented in the patient’s medical record.

All obstetric patients requiring surgical anesthesia are at risk for pulmonary aspiration of gastric contents, particularly if airway difficulties are encountered (see Chapter 28 ). Because conversion from neuraxial to general anesthesia may be required for any patient, either before or during surgery, strategies must be adopted to minimize this risk. The ASA and the American College of Obstetricians and Gynecologists recommendations allow modest amounts of clear liquids throughout an uncomplicated labor, with the avoidance of solid foods in laboring women. Clear liquids and solids are allowed up to 2 hours and 6 to 8 hours, respectively, before an elective operative procedure. The UK National Institute for Health and Clinical Excellence allows more liberal policies on oral intake in labor and suggests that women be allowed to drink isotonic fluids in labor and eat a light diet unless they develop risk factors that make general anesthesia more likely. All laboring women should be assessed and oral intake restricted if surgical intervention appears likely.

Once surgical intervention is required, antacids such as intravenous histamine-2 (H ₂ )-receptor antagonists or proton-pump inhibitors should be administered (if not recently administered), in case general anesthesia is necessary. However, these drugs take up to 30 minutes to become effective. If emergency general anesthesia is required, oral administration of a nonparticulate antacid such as sodium citrate is used to increase the pH of gastric contents. A dose of sodium citrate (0.3 molar) 30 mL is effective for approximately 30 minutes and should be administered shortly before the induction of general anesthesia.

Metoclopramide promotes gastric emptying and increases lower esophageal sphincter tone, although its efficacy is decreased by concurrent use of opioids. It may be given orally in labor or intravenously before general anesthesia.

A number of deaths and complications from presumed aspiration after tracheal extubation have been reported in the UK Confidential Enquiries into Maternal Deaths as well as the National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. When general anesthesia is administered to a woman with a potentially full stomach, reports have recommended that consideration be given to passing an “in and out” orogastric tube before extubation, and delaying extubation until the patient is awake and protective airway reflexes have returned.

The optimal laryngoscopic view yielding the best chance for successful intubation requires appropriate patient positioning. The sniffing position, with 35 degrees of neck flexion and 15 degrees of head extension, has been considered the ideal position for facilitating a view of the glottis by aligning the oral, pharyngeal, and laryngeal axes ( Fig. 29.6 ). In pregnancy, a ramped sniffing position, where the external auditory meatus and sternum are in horizontal alignment, optimizes first pass airway management success ( Fig. 29.7 ). The anteroposterior chest diameter is increased in obese patients, making 35 degrees of neck flexion unachievable in the supine position. Consequently, the shoulders and upper torso need to be raised; this can be achieved with the use of multiple blankets or pillows, or one of the many commercially available, wedge-shaped positioning cushions. Optimal elevation is verified by checking that the external auditory meatus and sternoclavicular joint are in horizontal alignment. Elevating the back of the operating table by 25 degrees may make laryngoscopy easier and also aids preoxygenation by increasing FRC and delaying time to oxygen desaturation. Some video laryngoscopes (see later discussion) do not require the patient’s head and neck to be in the sniffing position to obtain a view of the glottic opening. Nevertheless, even with video laryngoscopy, the ramped sniffing position facilitates oral insertion and manipulation of airway equipment to ensure timely intubation, particularly for the patient with obesity and macromastia.

Head and Neck Position During Laryngoscopy.

As the head position changes from neutral, the alignment of the oral axis (OA), pharyngeal axis (PA), and laryngeal axis (LA) changes within the upper airway. (A) The head is resting on a large pad that flexes the neck on the chest and aligns the LA with the PA (the neutral position). (B) The head is resting on a pad (which flexes the neck on the chest) and concomitant extension of the head on the neck can be seen, which brings all three axes into alignment (the sniffing position). (C) Extension of the head on the neck without concomitant elevation of the head on a pad, which results in nonalignment of the PA and LA with the OA.

From Benumof JL. Conventional [laryngoscopic] orotracheal and nasotracheal intubation [single-lumen type]. In: Benumof JL, ed. Clinical Procedures in Anesthesia and Intensive Care . JB Lippincott; 1991:115–148.

A morbidly obese patient is in an optimal position for direct laryngoscopy when an imaginary horizontal line can be drawn from the sternal notch through (or slightly anterior to) the external auditory meatus. To achieve this, the upper back and shoulders should be significantly elevated with pads or blankets (or a commercial elevation wedge/pillow) to allow the head to be extended at the atlanto-occipital joint. Additional blankets should be used to support the head in this position.

Left uterine displacement to minimize aortocaval compression should be maintained during preparation for, and induction of, general anesthesia. This may be achieved by tilting the operating table or by placing a wedge under the right hip.

In pregnancy, the decrease in FRC and increase in oxygen requirement result in rapid oxygen desaturation during periods of apnea (e.g., during induction of general anesthesia). The FRC is the primary reservoir for oxygen during apnea. Therefore, effective denitrogenation, or preoxygenation, of the FRC is vital to delay the onset of hypoxemia. Some authors suggest that the end-tidal oxygen fraction (F eto ₂ ) should be greater than 0.8 before anesthesia is induced.

The standard technique for preoxygenation has been to breathe 100% oxygen through a tight-fitting face mask at normal tidal volumes for 3 to 5 minutes. Given the urgent nature of obstetric general anesthesia, attention has focused on whether several maximal deep breaths over a shorter period can be as effective. Chiron et al. compared a traditional 3-minute technique with either eight deep breaths over 1 minute (8 DB/1 min) or four deep breaths over 30 seconds (4 DB/30 s). By monitoring the F eto ₂ , a marker of lung denitrogenation, the authors found that both the 3 minutes of tidal volume breathing and the 8 DB/1 min technique were more effective than the 4 DB/30 s technique. They suggested using the 8 DB/1 min technique in the setting of emergency obstetric anesthesia.

The use of maximal deep breaths to achieve denitrogenation may cause maternal hypocarbia and, therefore, should be limited to situations in which time does not allow normal tidal volume breathing of 100% oxygen to maximize oxygen storage in tissue and vascular body compartments. Indeed, during apnea, the time to desaturation depends on (1) the amount of oxygen stored in the lungs, tissue, and blood; (2) the mixed venous oxyhemoglobin saturation; and (3) the presence of intrapulmonary shunting.

A tight-fitting face mask is necessary to prevent room air entrainment, which reduces the efficiency of preoxygenation. With normal tidal volume breathing, preoxygenation is best achieved with oxygen flow rates in excess of 10 L/min for 3 minutes, although this may still be inadequate due to air entrainment. A 20-degree to 30-degree head-up tilt increases the FRC and delays the time to desaturation, especially in obese patients.

There has been recent interest in the use of high-flow nasal oxygen (HFNO) to prolong time to desaturation. Using flow rates of up to 70 L/min of oxygen, desaturation was not observed despite a mean apnea time of 248 seconds in nonobstetric patients undergoing rapid-sequence induction of anesthesia. Computer modeling of the effects of HFNO in the pregnant patient has shown that with an F io ₂ of 1.0 and a patent airway, apnea times in excess of 30 minutes are predicted without maternal desaturation to less than 90%. However, long apnea times should be avoided as much as possible due to a concomitant increase in Pa co ₂ and a fall in pH. It has also been suggested that HFNO may be used for preoxygenation of pregnant women. However, a prospective, observational study of 73 pregnant women found that a HFNO protocol of 30 L/min for 30 seconds, then 50 L/min for the next 150 seconds, was inadequate for preoxygenation (i.e., F eto ₂ ≥ 0.9) compared to face mask preoxygenation. With this protocol, only 60% of women achieved F eto ₂ ≥ 0.9 after 3 minutes. Further work is required to determine the role of HFNO in obstetric airway management.

In an attempt to minimize the risk for aspiration, rapid-sequence induction has become the standard technique for induction of obstetric general anesthesia. It usually consists of preoxygenation, rapid intravenous injection of a predetermined dose of induction agent followed immediately by succinylcholine administration, application of cricoid pressure, and avoidance of positive-pressure ventilation before tracheal intubation with a cuffed ETT. The urgency of intubation, coupled with avoidance of ventilation, may increase the likelihood of intubation failure.

Although in widespread use, the implementation of rapid-sequence induction is not uniform. Ideally, induction agents should provide rapid loss of consciousness, minimal hemodynamic instability, and optimal quality of intubating conditions. Propofol is a frequently used induction agent (see Chapter 26 ) and provides excellent suppression of pharyngeal and laryngeal reflexes. Opioids have traditionally not been part of rapid-sequence induction because of concerns about respiratory depression should intubation fail. Moreover, in the obstetric population, the potential for neonatal depression is greater if opioids are used, given rapid placental transfer of these drugs.

Because of its rapid onset, succinylcholine has traditionally been the muscle relaxant of choice for rapid-sequence induction of anesthesia in obstetric patients. Succinylcholine is associated with several undesirable side effects, most notably the potential for triggering malignant hyperthermia and anaphylaxis, and a prolonged duration of action in patients with cholinesterase deficiency. Despite reduced levels of plasma pseudocholinesterase in pregnancy, the duration of action of succinylcholine remains clinically unchanged in the obstetric patient. The ideal dose of succinylcholine, traditionally 1 mg/kg, remains controversial ; when combined with opioids in nonobstetric patients, succinylcholine 1 mg/kg fails to produce good intubating conditions at 1 minute in up to 8% of cases. Naguib et al. found that succinylcholine doses as high as 2 mg/kg still do not guarantee excellent intubating conditions in all patients; however, the authors found little extra benefit from using doses above 1.5 mg/kg. In a study of succinylcholine in nonobstetric obese patients, a dose of 1 mg/kg total body weight was found to provide more predictable laryngoscopic conditions than 1 mg/kg ideal or lean body weight.

The potential disadvantage of an increased succinylcholine dose is delayed return of spontaneous respiration, which is vital should intubation fail. Reducing the succinylcholine dose to 0.5 mg/kg slightly shortens recovery time and does not appear to compromise intubating conditions when administered with propofol and fentanyl in the nonobstetric population. In pregnancy, however, the reduced FRC and increased oxygen consumption make significant desaturation likely before the return of spontaneous respiration, no matter the succinylcholine dose. Because opioids are not commonly administered at induction, continued use of succinylcholine 1 to 1.5 mg/kg is recommended.

The use of alternative muscle relaxants for rapid-sequence intubation may be considered given the side-effect profile of succinylcholine. Rocuronium is often used when succinylcholine is contraindicated. In a study of 240 women undergoing rapid-sequence induction of general anesthesia for cesarean delivery, rocuronium did not prolong time to tracheal intubation, and it provided less resistance to laryngoscopy and a lower incidence of postoperative myalgia than succinylcholine. However, its prolonged duration of action is a significant concern when failure to ventilate or intubate occurs. Sugammadex , with its ability to rapidly reverse the effects of rocuronium, may address this concern (see earlier discussion). Authors of a 2015 metaanalysis concluded that succinylcholine (≥1 mg/kg), when used with propofol for rapid-sequence induction, produced superior intubating conditions than rocuronium (0.6 to 0.7 mg/kg) (relative risk, 0.86; 95% CI, 0.81 to 0.92). However, no significant difference was observed between the two agents when a larger dose of rocuronium (1.2 mg/kg) was used.

Although the effectiveness of cricoid pressure in preventing pulmonary aspiration of gastric contents has been challenged, it is frequently used during the induction of obstetric general anesthesia (see Chapter 26 , Chapter 28 ). However, the use of cricoid pressure can adversely affect the ease of ventilation, laryngoscopy, and intubation. In a comparison of cricoid pressure with 20, 30, and 44 N of force, increasing pressure was more likely to lead to cricoid deformity and esophageal occlusion, particularly in women. Difficulty with ventilation is less likely when 30 N is applied (currently accepted practice) than with 44 N (the previously suggested optimum value). When correctly applied, with an increase in force from 10 to 30 N with the induction of general anesthesia, there is little evidence of harm. However, when difficulty with intubation or ventilation is encountered, cricoid pressure may need to be adjusted, reduced, or released (see later discussion).

Although the cricoid pressure technique originally described by Sellick was a one-handed technique, the placement of a second hand behind the patient’s neck to prevent excessive neck flexion has been observed to provide a superior laryngoscopic view. However, the two-handed cricoid pressure technique does not allow the assistant to aid in other procedures, such as holding additional equipment necessary for difficult airway management.

Finally, although the traditional rapid-sequence induction technique avoids mask ventilation (to avoid introducing air into the stomach), this contributes to oxygen desaturation during the period of apnea. The current Obstetric Anaesthetists’ Association (OAA)/Difficult Airway Society (DAS) algorithm suggests that face mask ventilation (limiting P _max to 20 cm H ₂ O) can be considered.

The approach to the difficult airway in the obstetric patient depends on the clinical situation as well as the skill set of the anesthesia provider. A suggested approach for management of obstetric patients with an anticipated difficult airway is outlined in Fig. 29.8 . After an initial assessment of the patient, an airway management plan should be created and shared with the patient and other members of the multidisciplinary team. In extreme cases, anesthetic considerations may influence the mode and timing of delivery. Despite a thorough airway assessment and management plan, unanticipated or unrecognized airway issues and complications may arise; alternative algorithms and equipment should be readily available to ensure oxygenation and ventilation. Emergence and extubation should also be planned in advance. Lack of forethought and planning can lead to poor decision-making in crisis situations.

Algorithm for Anticipated Difficult Airway.

This algorithm is not intended to provide comprehensive guidance that addresses every contingency. Rather, it should help anesthesia providers consider the various options that are available. Management should be individualized, and the anesthesia provider’s clinical skills and judgment should guide decision-making. For additional information, the reader is referred to the American Society of Anesthesiologists practice guidelines. CSA , Continuous spinal anesthesia technique; CSE , combined spinal-epidural technique; LEA , lumbar epidural analgesia or anesthesia technique; spinal , spinal anesthesia technique.

The value of establishing and confirming a functional epidural catheter during labor in patients with an anticipated difficult airway has been described (see earlier discussion). Neuraxial anesthesia may also be preferable in patients with an anticipated difficult airway undergoing urgent or elective cesarean delivery. The choice of anesthetic technique (e.g., single-shot spinal, epidural, combined spinal-epidural [CSE], sequential CSE, continuous spinal techniques) depends on the circumstances and preferences of the anesthesia provider. Neuraxial techniques do not obviate the necessity of planning for airway management, as both inadequate neuraxial anesthesia and high spinal anesthesia may complicate any neuraxial technique and necessitate urgent airway intervention. Therefore, plans for securing the airway must always be preformulated, and standard and alternative airway equipment should be readily available.

Performing an awake intubation may be the safest option for the patient with an anticipated difficult airway, particularly if difficult face mask ventilation is anticipated or if neuraxial anesthesia is contraindicated or fails. Induction of general anesthesia with paralysis can distort airway anatomy by allowing soft tissue relaxation and movement of the larynx in an anterior direction, making attempts at direct laryngoscopy more difficult. Therefore, it may be optimal to secure the airway of these patients while they are awake and spontaneously breathing, before induction of general anesthesia. In skilled hands, the awake intubation can be accomplished quickly and comfortably with a high success rate.

Awake intubation can be performed with a variety of airway management devices, but the flexible fiberoptic bronchoscope offers unique advantages ( Box 29.2 ). Proper planning and execution, with attention to detail, are key to a high success rate. It is useful to have two anesthesia providers: one to perform the endoscopy and another to monitor the patient. Pulse oximetry, capnography, continuous electrocardiography, and blood pressure monitoring are mandatory. The level of conscious sedation must be constantly monitored to obtain the desired level for the procedure (see later discussion). Supplemental oxygen should be administered, with HFNO (see discussion above) as an optional adjunctive oxygenation technique.

When possible, an unhurried, thorough explanation of the technique to the patient helps allay anxiety. Pharmacologic premedication should include prophylaxis for pulmonary aspiration and an antisialagogue such as intravenous glycopyrrolate 0.2 mg. A dry mouth improves topical oral anesthesia by ensuring better contact between the local anesthetic and the mucosa. Secretions may also cause internal reflection from the light source and distort the fiberoptic view. Performing the procedure with the patient in the upright, rather than supine, position minimizes airway obstruction and aortocaval occlusion, enhances drainage of secretions, and allows better acceptance of topical anesthesia by the patient.

Topical Anesthesia

Providing adequate topical anesthesia of the upper respiratory tract is critical to a successful awake fiberoptic intubation. Local anesthetic agents can be used in two basic ways to provide topical upper airway anesthesia: direct application to the mucosa or the injection for laryngeal nerve blocks. Topical application of local anesthetic is the most commonly used technique owing to its ease and effectiveness. A variety of techniques can be successfully employed. For example, the patient can be asked to gargle and slowly swallow viscous lidocaine (2% or 4%), or lidocaine (2%, 4%, or 10%) can be aerosolized and sprayed onto the tongue and oropharynx.

Commercially available devices, which are produced in a variety of shapes and sizes, can aerosolize and spray local anesthetic solutions in a jetlike stream. The Mackenzie technique uses an intravenous cannula with an injection port (e.g., 20- or 18-gauge) connected to oxygen tubing via a three-way connector ( Fig. 29.9 ). Administration of local anesthetic from a syringe through the connector, with the oxygen flowing at 2 L/min, creates a jetlike spray. An additional method uses a nebulizer mask or mouthpiece, with 4% lidocaine (4 to 6 mL) placed in the nebulizer bowl and connected to an oxygen source at a flow rate of 8 L/min. This method is easy to administer, noninvasive, and comfortable for the patient, with minimal or absent coughing. Each of these techniques may be insufficient as a single entity and may be combined with other methods, including instillation of local anesthetic through the working channel of the fiberoptic bronchoscope channel.

Topical Airway Anesthesia.

The MacKenzie technique uses a 20-gauge intravenous cannula with an injection port connected to oxygen tubing via a three-way connector to produce a jetlike spray of local anesthetic administered from a syringe connected to the cannula with the oxygen flowing at 2 L/min.

The “spray as you go” (SAYGO) technique uses the working channel of the fiberoptic bronchoscope to instill local anesthetic onto the mucous membranes of the airway. The working channel of an intubating fiberoptic bronchoscope, such as the Olympus LF-2 (Olympus America Inc., Centre Valley, PA), is 600 mm long and 1.5 mm in diameter. If a small syringe is directly attached to the working channel port and the solution is merely injected, the local anesthetic is likely to stay in the channel rather than be sprayed onto the mucosa. This problem can be overcome by placing an epidural catheter through the working channel; using a Luer-Lok connector for the epidural catheter allows a direct and tight connection with the local anesthetic syringe and avoids leakage. The local anesthetic agent is drawn up in a 2-mL syringe and “dripped” on the mucus membranes; this instillation can be better targeted if the distal tip of the epidural catheter is allowed to protrude approximately 1 cm from the tip of the fiberoptic bronchoscope.

Airway Anesthesia and Risk for Aspiration

Some anesthesia providers are concerned that local anesthesia of the larynx might obtund airway protective reflexes, particularly after a transtracheal block. However, in a series of 129 patients, both the transtracheal injection and SAYGO techniques were effective and safe, with no evidence of regurgitation or aspiration in any patient. It has been suggested that topical anesthesia of the larynx does not impair voluntary motor function of the vocal cords, such as coughing on request, thus allowing the patient to protect their airway. The SAYGO technique may be preferred because the interval between topical anesthesia and endoscopy is minimal and if gastric reflux occurs during endoscopy it can be visualized and the gastric juice aspirated through the fiberoptic bronchoscope. The key to minimizing the risk for aspiration, however, is avoidance of oversedation. Nonetheless, administration of aspiration prophylaxis is advised, and the patient should be monitored for possible reflux or emesis.

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